taken 55 years ago, near to Wareham, Dorset, Great Britain

The signalman is holding up the hoop joined to the pouch containing the single-line token which the driver or fireman is about to collect from him. The token is issued from a machine in the signal box and is the driver's authority to proceed towards Swanage. With the token issued, no others can be taken out and ensures the train won't collide with another one! SY9086 : Worgret Junction - dropping off the single-line token shows how the token is returned.
(See Link for more information on the single-line token system.)